1. Field of the Invention
The invention is generally related to methods and processes for analyzing well production data and maximizing efficiency of reservoir production therefrom and is specifically directed to the evaluation of multilayer commingled reservoirs using commingled production data and production logging information.
2. Discussion of the Prior Art
Field production performance data and multiple pressure transient tests over a period of time for oil and gas wells in geopressured reservoirs have been found to often exhibit marked changes in reservoir effective permeability over the producing life of the wells. Similarly, the use of quantitative fractured well diagnostics to evaluate the production performance of hydraulically fracture wells have clearly shown that effective fracture half-length and conductivity can be dramatically reduced over the producing life of the wells. A thorough investigation of this topic may be found in the paper presented by Bobby D. Poe, the inventor of the subject application, entitled: “Evaluation of Reservoir and Hydraulic Fracture Properties in Geopressure Reservoir,” Society of Petroleum Engineers, SPE 64732.
Some of the earliest references to the fact that subterranean reservoirs do not always behave as rigid and non-deformable bodies of porous media may be found in the groundwater literature, see for example, “Compressibility and Elasticity of Artesian Aquifers,” by O. E. Meinzer, Econ. Geol. (1928) 23, 263–271. and “Engineering Hydraulics,” by C. E. Jacob, John Wiley and Sons, Inc. New York (1950) 321–386.
The observations of early experimental and numerical studies of the effects of stress-dependent reservoir properties demonstrated that low permeability formations exhibit a proportionally greater reduction in permeability than high permeability formations. The stress-dependence of reservoir permeability and fracture conductivity over the practical producing life of low permeability geopressured reservoirs has resulted in the following observations:                1. Field evidence of reservoir effective permeability degradation with even short production time can often be observed in geopressured reservoirs.        2. Quantitative evaluation of the field production performance of hydraulic fractures in both normal and geopressured reservoirs have resulted in the observation that the fracture conductivity of hydraulically fractured wells commonly decreases with production time.        3. Multiphase fracture flow has been demonstrated to dramatically reduce the effective conductivity of fractures.        4. Pre-fracture estimates of formation effective permeability derived from pressure transient tests or production analyses are often not representative of the reservoir effective permeability exhibited in the post-fracture production performance.        
The analysis of production data of wells to determine productivity has been used for almost fifty years in an effort to determine in advance what the response of a well will be to production-stimulation treatment. A discourse on early techniques may be found in the paper presented by R. E. Gladfelter, entitled “Selecting Wells Which Will Respond to Production-Simulation Treatment,” Drilling and Production Procedures, API (American Petroleum Institute), Dallas, Tex., 117–129 (1955). The pressure-transient solution of the diffusivity equation describing oil and gas flow in the reservoir is commonly used, in which the flow rate normalized pressure drops are given by:(Pi−Pwf)/qo, and{Pp(Pi)−Pp(Pwf)}/qg,for oil and gas reservoir analyses, respectively, wherein:                Pi is the initial reservoir pressure (psia),        Pwf is the sandface flowing pressure (psia)        qo is the oil flow rate (STB/D)        Pp is the pseudopressure function, psia2/cp and        qg is the gas flow rate (Mcsf/D).        
While analysis of production data using flow rate normalized pressures and the pressure transient solutions work reasonably well during the infinite-acting radial flow regime of unfractured wells, boundary flow results have indicated that the production normalization follows an exponential trend rather than the logarithmic unit slope exhibited during the pseudosteady state flow regime of the pressure-transient solution.
Throughout most of the production history of a well, a terminal pressure is imposed on the operating system, whether it is the separator operating pressure, sales line pressure, or even atmospheric pressure at the stock tank. In any of these cases, the inner boundary condition is a Dirichlet condition (specified terminal pressure). Whether the terminal pressure inner boundary condition is specified at some point in the surface facilities or at the sandface, the inner boundary condition is Dirichlet and the rate-transient solutions are typically used. It is also well known that at late production times the inner boundary condition at the bottom of the well bore is generally more closely approximated with a constant bottomhole flowing pressure rather than a constant rate inner boundary condition.
An additional problem that arises in the use of pressure-transient solutions as the basis for the analysis of production data is the quantity of noise inherent in the data. The use of pressure derivative functions to reduce the uniqueness problems associated with production data analysis of fractured wells during the early fracture transient behavior even further magnifies the effects of noise in the data, commonly requiring smoothing of the derivatives necessary at the least or making the data uninterpretable at the worst.
There have been numerous attempts to develop more meaningful production data analyses in an effort to maximize the production level of fractured wells. One such example is shown and described in U.S. Pat. No. 5,960,369 issued to B. H. Samaroo, describing a production profile predictor method for a well having more than one completion wherein the process is applied to each completion provided that the well can produce from any of a plurality of zones or in the event of multiple zone production, the production is commingled.
From the foregoing, it can be determined that production of fractured wells could be enhanced if production performance could be properly utilized to determine fracture efficiency. However, to date no reliable method for generating meaningful data has been devised. The examples of the prior art are at best speculative and have produced unpredictable and inaccurate results.